Process for the bleaching of paper pulp

ABSTRACT

Process for the bleaching of paper pulp in which an aqueous suspension of paper pulp is subjected to a bleaching treatment with hydrogen peroxide in the presence of magnesium hydroxide and of at least one poly-α-hydroxyacrylic acid, its salt or their mixture.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national stage application under 35 U.S.C.371 of International Application PCT/EP2009/056278, which claims thebenefit of the European application No. 08156934.5 filed on May 26,2008, the whole content of this application being herein incorporated byreference for all purposes.

TECHNICAL FIELD

The present invention relates to a process for the bleaching of paperpulp, especially mechanical paper pulp, through a bleaching treatmentwith hydrogen peroxide and an alkali source. Such bleaching treatmentusually involves the use of peroxide stabilizers.

BACKGROUND

It is known to bleach mechanical paper pulp with oxidizing agents suchas hydrogen peroxide in an alkaline medium and in the presence of aperoxide stabilizer.

For instance, the U.S. Pat. No. 6,221,209 of SOLVAY INTEROX relates to aprocess for bleaching a chemical paper pulp by first purifying the pulpso as to reduce its manganese content and then by bleaching it withhydrogen peroxide in alkaline medium, especially at a pH adjusted to atleast 10 by the addition of an alkaline compound (i.e. sodiumhydroxide), and in the presence of a stabilizing agent, namely sodiumsilicate.

It is also known to use other alkaline sources than sodium hydroxide,such as magnesium hydroxide. For example, the U.S. Pat. No. 7,052,578discloses the use of magnesium hydroxide for the hydrogen peroxidebleaching of mechanical paper pulp.

However, even if, according to U.S. Pat. No. 7,052,578, it is possibleto reach high brightness results using magnesium hydroxide alone asalkaline source, it is more difficult compared to the use of sodiumhydroxide and sodium silicate as it requires an extended reaction time,an increased peroxide charge, and the addition of chelating agents suchas DTPA, even if a chelating step was conducted prior to the bleachingtreatment.

When magnesium hydroxide is used as alkaline source in the hydrogenperoxide bleaching of mechanical paper pulp, it is difficult to achievea satisfactory brightness, and the consumption of hydrogen peroxide fora given brightness is high.

SUMMARY

The purpose of the present invention is to provide a process which doesnot present the above disadvantages and which enables to obtain a higherpaper pulp brightness compared to the known use of other alkalinesources than sodium hydroxide, such as magnesium hydroxide or calciumhydroxide. The purpose of the present invention is also to provide aprocess which allows decreasing the consumption of hydrogen peroxide fora given brightness.

The present invention therefore relates to a process for the bleachingof paper pulp in which an aqueous suspension of paper pulp is subjectedto a bleaching treatment with hydrogen peroxide in the presence of atleast one alkaline earth hydroxide selected from magnesium hydroxide andcalcium hydroxide and of at least one poly-α-hydroxyacrylic acid, itssalt or their mixture.

BRIEF DESCRIPTION OF THE DRAWING

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawing inwhich:

FIG. 1 illustrates the influence of the addition of sodiumpoly-α-hydroxyacrylate (PHAS) during the bleaching of a thermomechanicalpaper pulp with hydrogen peroxide and in the presence of magnesiumhydroxide.

DETAILED DESCRIPTION

Indeed, it has surprisingly been found that, in the presence of at leastone poly-α-hydroxyacrylic acid, its salt or their mixture, it ispossible to reach higher brightness, or to decrease the hydrogenperoxide consumption for a given brightness, when at least part of thealkaline compound is selected from alkaline earth hydroxides andespecially from magnesium hydroxide and calcium hydroxide.

Furthermore, sodium silicate cannot be used when using alkaline earthhydroxides such as magnesium hydroxide and/or calcium hydroxide asalkaline source, because it forms precipitates with those products,leading to scaling. Such scaling creates problems in the wholeproduction line including in the paper machine, when the mill is anintegrated site. Thus, when alkaline earth hydroxides such as magnesiumhydroxide and/or calcium hydroxide are used, the common practice is tobleach without any stabilizer, except aminopolycarboxylic acids or theirsalts, such as diethylene triamine penta-acetic acid (DTPA) or itssodium salt which will act as chelating agents and will therefore limitthe decomposition of the hydrogen peroxide which is catalyzed bytransition metals such as manganese or iron. Nevertheless, theconditions present during the bleaching are usually not suitable for agood efficiency of such chelating agents and therefore the effect ofsuch stabilizers is very limited.

One of the essential features of the present invention resides in thecombined use of an alkaline earth hydroxide selected from magnesiumhydroxide and calcium hydroxide as alkaline source (referred to as“alkaline earth hydroxide according to the invention”) and ofpoly-α-hydroxyacrylic acid, its salt or their mixtures (referred to asPHAA) as hydrogen peroxide stabilizer, during the hydrogen peroxidebleaching of paper pulp.

The PHAA used in the process of the invention generally has a molecularweight within a specific range. As the molecular weight is ratherdifficult to measure, it is inferred by measuring the viscosity, thehigher the molecular weight, the higher the viscosity. Viscosity in therange of 10 to 100 mPa·s is usual but more particularly it can be in therange of 20 to 60 mPa·s. Commonly, the PHAA is used as a sodium salt ofpoly-α-hydroxyacrylic acid, corresponding to sodiumpoly-α-hydroxyacrylate (referred to as PHAS). In the invention, goodresults are obtained using only one PHAA. Nevertheless, more than onePHAA can be used.

The bleaching treatment according to the present invention is generallycarried out at an initial pH of at least 8, in particular of at least8.5, values of at least 9 being preferred. The initial pH is usually atmost 14, in special cases at most 13, values of at most 12 beingconvenient. The final pH of the aqueous paper pulp suspension, at theend of the bleaching treatment is in general at least 6, especially atleast 7, in particular at least 8. The final pH of the aqueous paperpulp suspension at the end of the bleaching treatment is in many casesat most 11, particularly at most 10. The pH of the aqueous suspensionbefore, during or after the bleaching treatment is measured usingequipment normally found in pulp mills for such a purpose. According tothe invention, the natural pH of the paper pulp suspension is adjustedto the required value by means of an alkaline compound comprising atleast one alkaline earth hydroxide selected from magnesium hydroxide andcalcium hydroxide.

The amount of alkaline earth hydroxide according to the invention, usedin the bleaching step of the present invention, is generally such thatthe pH of the suspension is maintained within the above-mentioned range.This amount is usually at least 0.1% by weight of dry pulp, preferablyat least 1% by weight of dry pulp, more preferably at least 2% by weightof dry pulp, especially above 2% by weight of dry pulp. The amount ofalkaline earth hydroxide according to the invention, used in thebleaching step of the present invention, is generally at most 10% byweight of dry pulp, with particular preference at most 5% by weight ofdry pulp. The amount of alkaline earth hydroxide according to theinvention, used in the bleaching step of the present invention, is forinstance of from 2 to 5% by weight of dry pulp, for example about 3% byweight of dry pulp.

The amount of alkaline earth hydroxide according to the inventiondepends on the amount of hydrogen peroxide used and on the type of pulp.The ratio alkaline earth hydroxide according to the invention/hydrogenperoxide can for example vary from 0.3 to 2.5, preferably from 0.5 to2.0 depending on the amount of hydrogen peroxide added and on the typeof pulp. For example, the ratio may be about 1.

The alkaline earth hydroxide according to the invention is usually addedto the paper pulp suspension in the form of an aqueous slurry.Preferably, the particle size of the alkaline earth hydroxide accordingto the invention is sufficiently small for the slurry to be readilysuspendible, advantageously having a mean particle size of less thanabout 10 μm, preferably less than 6 μm, for example about 2 μm.

The bleaching step of the invention is generally carried out at aconsistency of at least 10% of dry pulp based on the weight of pulpsuspension. The efficiency of the process increases as the consistencyis increased up to a value of around 30% by weight of dry pulp. Theconsistency can be up to 40% by weight of dry pulp.

The bleaching treatment of the invention is usually carried out using anamount of hydrogen peroxide required to achieve the final targetbrightness. The amount of hydrogen peroxide is generally equal to orhigher than 0.1% by weight of dry pulp, preferably equal to or higherthan 0.5% by weight of dry pulp, more preferably equal to or higher than1% by weight of dry pulp. The amount of hydrogen peroxide is usuallyequal to or lower than 8% by weight of dry pulp, especially equal to orlower than 6% by weight of dry pulp, and more particularly equal to orlower than 5% by weight of dry pulp. For example, the amount of hydrogenperoxide may be about 2 to 4% by weight of dry pulp.

The bleaching treatment of the invention is usually carried out at atemperature equal to or higher than 40° C., especially equal to orhigher than 60° C. The temperature can be up to about 100° C. Thetemperature is preferably less than 100° C., more preferably equal to orless than 95° C., for example equal to or less than 85° C. The optimumbleach temperature depends on the wood type and pulping process but isnormally in the range of 60 to 95° C.

The duration of the bleaching treatment of the process of the inventionis usually from 10 min to 6 hours, varying from mill to mill. Forinstance, it can vary from 30 to 300 minutes.

For example, the bleaching treatment can be carried out at a pH of from8 to 14, at a consistency of from 10 to 40%, and at a temperature offrom 60 to 95° C., during 1 to 4 hours.

The amount of PHAA, especially PHAS, used in the bleaching step of theinvention is usually equal to or higher than 0.05% by weight of drypulp, especially equal to or higher than 0.1% by weight of dry pulp. Theamount of PHAA is commonly equal to or lower than 2% by weight of drypulp, values of at most 1% being suitable. For example, typical amountsof PHAA are about 0.1 to 0.5% by weight of dry pulp. The optimum amountis dependent on the amount of metallic ions in the pulp and must beoptimized for each particular mill.

The bleaching step of the invention can be carried out in the presenceof any other alkaline or alkaline earth hydroxides than the alkalineearth hydroxides according to the invention, such as in the presence ofsodium hydroxide. The bleaching step of the invention may also becarried out in the absence sodium hydroxide. The bleaching step of theinvention can also be carried out in the presence of any otheradditives, such as magnesium sulfate or other soluble magnesium salts.Preferably, the bleaching step is carried out in the absence of anyother stabilizer, and especially in the absence of sodium silicate.

In a preferred embodiment, prior to the bleaching treatment, the aqueoussuspension of the paper pulp may be subjected to a chelating treatmentto reduce the metallic ion content of the suspension, and especially itsmanganese content. It is for example usual to carry out a pretreatmentstep (or chelating treatment), with at least one aminopolycarboxylicacid, its salt or their mixture (referred to as APCA) prior to thebleaching treatment.

The APCA used in the pretreatment step is generally chosen from ethylenediamine tetra-acetic acid (EDTA), diethylene triamine penta-acetic acid(DTPA), triethylene tetramine hexa-acetic acid (TTHA), cyclohexanediamine tetra-acetic acid (CDTA), methylglycine di-acetic acid (MGDA),nitrilo tri-acetic acid (NTA). The APCA may be used as the free acid,its salt or their mixtures. The APCA is preferably used as its salt. Thesalt is usually the alkali metal salt such as sodium or potassium or theammonium salt or a mixture thereof. Sodium salts give good results. Themost preferred APCA is the sodium salt of DTPA. Good results aregenerally obtained using only one APCA. Nevertheless, more than one APCAcan be used. The chelating pretreatment step may be conducted asdescribed in the International patent application filed asEP2008/052056, the text of which is incorporated herein by reference.

In a variant of the invention, the bleaching treatment may be carriedout in the presence of at least one chelating agent, in particular of atleast one APCA, such as those disclosed above. According to such avariant, the amount of APCA used in the bleaching treatment of theinvention is usually at least 0.05% wt based on the weight of dry pulp.The amount of APCA is commonly at most about 1% wt based on the weightof dry pulp, values of at most 0.8% wt being suitable.

If at least one APCA is present during the bleaching treatment of theinvention, the weight ratio between PHAA, especially PHAS, and APCA isusually at least 1:10, preferably at least 1:5, ratios of at least 1:3being satisfactory. The weight ratio PHAA/APCA, especially PHAS/APCA, isgenerally at most 10:1, especially at most 5:1, values of at most 3:1giving the best results.

In another variant of the invention, the bleaching treatment is carriedout in the absence of a chelating agent or of another stabilizer thanPHAA.

The paper pulp treated in the present invention can be chosen fromchemical paper pulps, mechanical paper pulps or recycled paper pulps.The best results are obtained with mechanical paper pulps. By mechanicalpaper pulps are meant paper pulps obtained by mechanical treatment.Examples of such paper pulps are pressure groundwood (PGW), stonegroundwood (SGW), thermomechanical pulp (TMP), refiner mechanical pulp(RMP), chemithermomechanical pulp (CTMP) and alkaline peroxidemechanical pulp (APMP or APP). The present invention therefore alsorelates to the bleaching process of the invention applied to mechanicalpaper pulp.

The process of the invention can further comprise other treatment stepssuch as one or more additional dilution step(s), one or more additionalchelating step(s), one or more additional bleaching step(s), one or morewashing step(s) and/or one or more extraction step(s).

It is recommended to have a good mixing of the paper pulp to be treatedwith the hydrogen peroxide, the alkaline earth hydroxide according tothe invention, and the PHAA. The order of addition of the alkaline earthhydroxide of the invention, the PHAA and the hydrogen peroxide is eitherfirst the PHAA, then the alkaline earth hydroxide of the invention, andlastly the hydrogen peroxide or first the alkaline earth hydroxide ofthe invention, then the PHAA, and lastly the hydrogen peroxide. Thosethree products may also be premixed all together or two of them can bepremixed. The alkaline earth of the invention is usually added as asuspension in water. Dilution water may also be added. Usually, thereactants are introduced in the paper pulp suspension via a pump and ahigh consistency mixer, especially when the bleaching is performed at aconsistency close to 30%. An example of configuration is: the chemicalsare added in the preheating screw of the HC mixer.

The present invention also relates to the use of at least one alkalineearth hydroxide selected from magnesium hydroxide and calcium hydroxideto control the pH of an aqueous suspension of paper pulp during thebleaching treatment of the paper pulp with hydrogen peroxide, whereinsaid bleaching treatment is conducted in the presence of at least onepoly-α-hydroxyacrylic acid, its salt or their mixture.

The present invention is further illustrated below without limiting thescope thereto.

EXAMPLES Examples 1 (comparative) and 2 (According to the Invention)

According to Example 1, a mechanical pulp produced by a TMP processusing spruce as raw material, at a consistency of about 15%, wasbleached at 80° C. during 3 hours. Hydrogen peroxide was added in anamount of 3% by weight of dry pulp. Magnesium hydroxide was added in anamount of 3% by weight of dry pulp. The brightness of the paper pulpobtained in these conditions was 75.1° ISO and the residual peroxidecontent after the bleaching treatment was 9% of the initial charge.

In Example 2, the same conditions as in Example 1 were applied exceptthat 0.5% of PHAS were added to the bleaching step. The resultingbrightness of the paper pulp was 76.6° ISO and the residual peroxidecontent was 17% of the initial charge.

Those two examples illustrate that, according to the invention, a higherbrightness is obtained when a PHAA is added during the hydrogen peroxidebleaching of paper pulp in the presence of magnesium hydroxide as basiccompound.

Examples 3 (Comparative) and 4 (According to the Invention)

In Example 3, a mechanical pulp produced by a TMP process using spruceas raw material, at a consistency of about 15%, was bleached at 70° C.during 2 hours. Hydrogen peroxide was added in an amount of 3% by weightof dry pulp. Magnesium hydroxide was added in an amount of 3% by weightof dry pulp. The brightness of the paper pulp obtained in theseconditions was 73.8° ISO and the residual peroxide content after thebleaching treatment was 27% of the initial charge.

In Example 4, the same mechanical pulp as in example 3 was bleached at70° C. during 2 hours in the presence of an amount of hydrogen peroxideof 2% by weight of dry pulp, of magnesium hydroxide of 3% by weight ofdry pulp, and of an amount of PHAS of 0.3% by weight of dry pulp. Theresulting brightness of the paper pulp was the same as in Example 3(73.8° ISO) and the residual peroxide content was 31% of the initialcharge.

Those two examples illustrate that, according to the invention, it ispossible to decrease the hydrogen peroxide consumption for a givenbrightness.

Examples 5 and 6 (Referring to FIG. 1)

In Example 5, a mechanical pulp produced by a TMP process using spruceas raw material, at a consistency of about 15%, was bleached at 70° C.during 2 hours. Hydrogen peroxide was added in an amount of 3% by weightof dry pulp. Magnesium hydroxide was added in an amount of 3% by weightof dry pulp. PHAS was added in an amount of 0 to 0.5% by weight of drypulp.

The same conditions as in Example 5 were reproduced in Example 6, exceptthat the amount of hydrogen peroxide was 2% by weight of dry pulp.

These results show that an amount of PHAS of 0.35% by weight of dry pulpcan reduce the hydrogen peroxide consumption by 1% by weight of dry pulpfor the same brightness. These results also show that for an amount ofhydrogen peroxide of 2% by weight of dry pulp, the brightness of theresulting paper pulp can be increased from 72.4 to 73.9° ISO by theaddition of an amount of PHAS of 0.35% by weight of dry pulp.

1. A process for the bleaching of paper pulp in which an aqueoussuspension of paper pulp is subjected to a bleaching treatment withhydrogen peroxide in the presence of at least one alkaline earthhydroxide selected from the group consisting of magnesium hydroxide andcalcium hydroxide and in the presence of at least onepoly-α-hydroxyacrylic acid, its salt or their mixture.
 2. The processaccording to claim 1, wherein the alkaline earth hydroxide is used in anamount of 0.1 to 10% by weight of dry pulp.
 3. The process according toclaim 1, wherein the bleaching step is carried out in the absence ofsodium hydroxide.
 4. The process according to claim 1, wherein thebleaching step is carried out in the absence of sodium silicate.
 5. Theprocess according to claim 1, wherein, prior to the bleaching treatment,the aqueous suspension of the paper pulp is first subjected to achelating treatment with at least one aminopolycarboxylic acid, its saltor their mixtures.
 6. The process according to claim 1, wherein thepoly-α-hydroxyacrylic acid is in the form of its sodium salt.
 7. Theprocess according to claim 1, wherein the bleaching treatment is carriedout at a temperature equal to or less than 100° C.
 8. The processaccording to claim 1, wherein the bleaching step is carried out at aninitial pH of from 8 to
 14. 9. The process according to claim 1, whereinthe final pH, at the end of the bleaching step, is of from 6 to
 11. 10.The process according to claim 1, wherein the duration of the bleachingstep is of from 1 to 6 hours.
 11. The process according to claim 1,wherein the hydrogen peroxide is used in an amount of 0.1 to 8% byweight of dry pulp.
 12. The process according to claim 1, wherein thepoly-α-hydroxyacrylic acid, its salt or their mixture is used in anamount of from 0.1 to 2.0% by weight of dry pulp.
 13. The processaccording to claim 2, wherein the aminopolycarboxylic acid, its salt ortheir mixture is selected from the group consisting of ethylene diaminetetra-acetic acid, diethylene triamine penta-acetic acid, triethylenetetramine hexa-acetic acid, cyclohexane diamine tetra-acetic acid,methylglycine di-acetic acid, nitrilo tri-acetic acid, their salts, andmixtures thereof.
 14. The process according to claim 1 applied to thebleaching of mechanical paper pulp.
 15. A method for controlling the pHof an aqueous suspension of paper pulp during the bleaching treatment ofthe paper pulp with hydrogen peroxide, wherein said bleaching treatmentis conducted in the presence of at least one poly-α-hydroxyacrylic acid,its salt or their mixture, said method comprising using at least onealkaline earth hydroxide selected from the group consisting of magnesiumhydroxide and calcium hydroxide.
 16. The process according to claim 1,wherein the bleaching treatment is carried out at a temperature from 60to 95° C.